The present invention relates to parallel optical interconnects using multimode fiber, particularly to wavelength division multiplexing (WDM) parallel optical fiber interconnects, and more particularly to a compact multiwavelength transmitter module for multimode parallel optical fiber ribbon cable.
Massively-parallel distributed computing systems, as well as the growth of multimedia and the Internet, are limited by ever-increasing data transmission capacity requirements. In these applications the in/out (I/O) bottleneck is particularly severe as the number of nodes in the network is scaled upwards. Optical fiber is clearly superior to electronic switching and cabling in terms of bandwidth, cross-talk, and interconnection fan-out. However, the inherent fiber bandwidth is vastly underutilized because of limits on the modulation rate of the laser diode transmitter used to convert the data stream from the electronic to the photonic domain.
Wavelength division multiplexing (WDM) is a means of encoding information in parallel onto multiple transmission wavelengths transmitted within a single optical fiber to better utilize this bandwidth. Another technique for enhancing bandwidth is simply to parallelize the physical transmission medium itself by forming the transmission link using multiple fibers, each fiber carrying a different xe2x80x9cbitxe2x80x9d in parallel. This is most cost effective for short-distance ( less than 100 m) links, such as those between multiple compute boxes in a distributed computing cluster or between a compute box and disk array, where the cost of the fiber is low compared to the transmitter or receiver modules. Key to achieving component cost reduction is the use of multi-mode fiber, which uses a fiber core diameter approximately 10 times larger than the single mode fiber used in long-distance telecom applications; the larger size greatly eases the cost of alignment and packaging. The ultimate in performance at low cost will be achieved by combining WDM with parallel multimode optical fiber links, achieving an Mxc3x97N fold improvement in bandwidth given M wavelengths and N fibers, while keeping cost in line by choosing an appropriate packaging architecture. Single wavelength systems using 12-wide fiber cable are commercially available from several sources, including Siemens in Germany, Optobahn and Vixel Corp. in the USA.
A parallel fiber multi-wavelength optical transmitter module is a necessary element in such a WDM system. Practically, this transmitter requires an Mxc3x97N array of emitters which can each be independently modulated. An important enabling technology is the vertical-cavity surface-emitting Laser (VCSEL). A VCSEL is a semiconductor laser diode which emits light perpendicular to the plane of the substrate on which it is fabricated and consists of an active layer residing within an optical cavity sandwiched by two distributed mirrors. Because of this vertical orientation, two-dimensional VCSEL arrays are easily fabricated. Furthermore, the emission wavelength is controlled by the cavity layer thickness and is thus an easily-controlled design parameter. However, the number of different wavelength channels achievable monolithically on a single substrate may be limited (ranging from 1 to 4 or 8).
Single chip (monolithic) approaches don""t provide sufficient wavelength range and can furthermore lead to problems with crosstalk. Such a single chip approach is exemplified by S. Hu et al., xe2x80x9cMultimode WDM Optical Data Links With Monolithically Integrated Multiple-Channel VCSEL and Photodetector Arrays,xe2x80x9d IEEE Journal of Quantum Electronics, Vol. 34, No. 8, pp. 1403-1414, August 1998, which described a method of coupling the light of multiple emitters into a single multi-mode fiber, each emitter lasing at a different wavelength, by fabricating the emitters in close physical proximity on the same semiconductor substrate.
The present invention involves a multiwavelength transmitter module suitable for multimode parallel optical fiber ribbon cable. A key feature of the invention is to compactly and efficiently combine the light from two or more clusters of optical emitters, each in a different wavelength, into a fiber ribbon. Another key feature of the invention is bringing together two emitter arrays fabricated on different substratesxe2x80x94each array designed for a different wavelengthxe2x80x94into close proximity.
It is an object of the present invention to provide a multi-wavelength transmitter module suitable for multimode parallel optical fiber ribbon cable.
A further object of the invention is to extend the bandwidth of transmitters using single wavelength systems by using wavelength division multiplexing (WDM).
Another object of the invention is to provide a compact multiwavelength transmitter module for multimode fiber optic ribbon cable.
Another object of the invention is to provide a means for coupling light from an Mxc3x97N array of emitters onto N fibers, where the M wavelengths may be distributed across two or more VCSEL chips, and combining multiplexers into a compact package.
Another object of the invention is to compactly and efficiently combine the light from two or more clusters of optical emitters into an optical fiber ribbon, with each being in a different wavelength band.
Another object of the invention involves bringing together two emitter arrays fabricated on different substrates into close physical proximity, each array having a different wavelength.
Another object of the invention is to provide a multiwavelength transmitter for multimode fiber optics with sufficient wavelength range and which prevents crosstalk.
The objects and advantages of the present invention will become apparent from the following description and accompanying drawings. Basically, the invention provides a compact multiwavelength transmitter module for multimode fiber optic ribbon cable, using wavelength division multiplexing (WDM). The multiwavelength transmitter of the invention includes five (5) novel features: 1) two-wavelength transmitter subunits, 2) connecting of the subunits with mechanically transferable (MT) ferules/guide pins, 3) fiber superstrate array containing wavelength-selective turning mirror/filters, 4) attaching VCSEL die to a silicon optical bench, and 5) combining the hybrid approach to transmitter integration with monolithic techniques which achieve multiple wavelengths on the same chip. Items 1-4 above can produce 2, 4, 6, 8, etc. wavelengths in a compact format (e.g., pin grid array package of xcx9c1 square inch array and 0.5 inch height). Additional improvements in number-of-wavelengths or compactness can be achieved using item 5 above.